The higher altitudes at which modern aircraft operate provide numerous advantages over lower flight plans. There are generally fewer clouds and less turbulence at high altitudes, and thus passengers are subjected to smoother flights. Aircraft engines are also optimized for high altitudes, so fuel efficiency is another benefit of operation at a higher altitude. Because the air is thinner, the aircraft meets with less resistance and the plane gets more lift with less thrust, improving fuel efficiency. Because the air is thinner and there is less moisture, there is less accumulation of ice at high altitudes as well.
For these reasons, aircraft have been flying at higher altitudes to reduce fuel consumption and enjoy the benefits of less weather and turbulence. However, one downside to flying at higher elevations is the presence of ozone, a highly flammable oxygen molecule found at higher concentrations in the upper atmosphere, and its potentially harmful effects. At higher elevations, the ambient O3 concentration becomes an unavoidable contaminant in both the cabin and the fuel system. Ozone enters the aircraft at high elevations through the air conditioning system or other venting orifices. Once on-board, ozone in the cabin causes numerous physical ailments to passengers and crew, including fatigue, headaches, shortness of breath, nausea (air sickness), sinus irritation, and in certain susceptible passengers, chest pains and pulmonary distress. Because of these adverse health effects, the Federal Aviation Administration (FAA) set limits for ozone content in passenger aircraft cabins. According to FAA AC-120-38, ozone must be less than 0.25 parts per million by volume (ppmv) at any given instant above 32,000 feet (FL-320). Above 27,000 feet (FL-270) for each flight segment that exceeds 4 hours, the time weighted average amount of ozone must be less than 0.1 ppmv.
To meet the FAA requirements, aircraft are equipped with catalytic devices that remove ozone from the environment. These converters ensure that oxygen and ozone concentrations are below the regulations even under a worse-case scenario. Catalytic ozone converters in most cases control the oxygen or ozone concentrations by oxidizing the unwanted gas. They typically consist of a metal housing for a precious metal catalyst, which is selected based upon the type of gas to be removed.
Current ozone converters, however, can also produce negative ramifications. In some converters using thermal decomposition, the energy used is so extensive that any fuel savings from altitude adjustment is rendered useless. The accumulation of particles on the adsorbent surface of the converter also decreases the efficiency of carbon adsorption filters, which leads to costly filter maintenance or replacement. Reactors in non-thermal plasma oxidation can contain ozone itself; therefore, a compromise between the required electrical power and the O3 generated must be reached. Also, if the VOCs are neither hydrogen- or carbon-based, non-thermal plasma oxidation may produce toxic substances, such as hydrochloric acid.
Ozone also corrodes various plastics and rubbers that are used on an aircraft. One critical component that is affected by the presence of ozone is the aircraft's air separation module (“ASM”). An air separation module separates atmospheric air into its primary components, typically nitrogen and oxygen, and in some cases argon and other rare inert gases. Membrane gas separation is used to provide oxygen poor and nitrogen rich gas instead of air to fill the fuel tanks of aircraft, thus greatly reducing the chances of accidental combustion in the fuel system. That is, to prevent the combustion of flammable materials unavoidably trapped in a fuel tank, a chemically non-reactive or “inert” gas, such as nitrogen, in introduced into the tank of the aircraft to force out the possibly reactive oxygen gas. As is well known, there are three elements that are required to initiate and sustain combustion: an ignition source (heat), fuel and oxygen. Combustion may be prevented by reducing any one of these three elements. If the presence of an ignition source cannot be prevented within an aircraft's fuel tank, then the tank may be made inert by: 1) reducing the oxygen concentration of the ullage (the space above a liquid fuel) to below that capable of combustion (the combustion threshold); 2) reducing the fuel concentration of the ullage to below the “lower explosive limit” (LEL), the minimum concentration capable combustion; or 3) increasing the fuel concentration to above the “upper explosive limit” (UEL), the maximum concentration capable of combustion.
Thus, as stated above, flammable vapors in fuel tanks are rendered inert by replacing the air, which may contain increased levels of ozone, in the tank with an inert gas, such as nitrogen, nitrogen enriched air, steam or carbon dioxide. This reduces the oxygen concentration of the ullage to below the combustion threshold. Alternate methods based on reducing the ullage fuel-air ratio to below the LFL or increasing the fuel-air ratio to above the UFL have also been proposed. Conversely, membrane gas separation is currently used to provide oxygen enriched air to pilots flying at great altitudes in aircraft without pressurized cabins.
Air separation modules that are needed to accomplish this gas exchange are made from a specialized plastic membrane which breaks down when exposed to ozone. The function of the ASM membrane is to remove the O2 oxygen from the air and leave the N2 “nitrogen rich” air which is essentially inert to be pumped into A/C fuel tanks. The ASM, and particularly the membrane, are a very expensive components, and when the membrane fails then elevated concentrations of O2 pass through, making ASM exit air less inert or more O2 rich, compromising the safety or inertness of the fuel tanks.
Airplane bleed airflow is a source of air used on most commercial aircraft. Bleed air typically can reach temperatures of 450° F., which creates several problems on this issue. One is the higher temperature leads to conversion of O2 to O3 and vice versa in the airflow. Thus, to more safely inert the system both O2 and O3 must be accounted for.
The airline industry has been looking for a high efficiency and a longer lifespan ozone converter to protect the expensive air separation modules. A solution must increase the efficiency of the existing ozone converters and account for the presence of both O2 to O3. The present invention is designed for this very purpose.